I'll bet there was a brass housing stop on the brake housing. Salt water, brass and aluminum do exactly what you saw. I'll bet also that the brass stop was near completely wasted away. I've spent years salt water sailing. On boats we take pains to see that bronze is used, not brass, that aluminum and bronze/brass do not touch and that important bronze fittings are protected by mechanically attaching a pies of sacrificial zinc which corrode away (like I presume your brass stop did) to protect the bronze.

The other approach is to keep salt water away. Not feasible in marine environments and not for everybody on a trainer.

Ben